For those that have a 99 F150 4.2L with similar idle/running rough issues.

Recently wrote my 95 Ranger off, long story short so i decided to get myself a bit bigger truck so i picked up a 99 F150, with a 4.2L in it (i'm a student gimme a break). Anyways it was idling pretty rough and after some investigating online i read a lot about: EGR, IAC, Dirty intake, and vacuum leaks. In the past spraying some brake clean or intake cleaner around suspect areas has proven really successful to find leaks as the idle changes immediately. But this time the idle was so sporadic on it own i couldn't determine if it was normal symptom or the spray changing the idle.

Symptoms:
-rough idle especially once warm
-a miss once in a while
-surging of the idle RPM on a sporadic 10 to 30 second period
-slightly less surging with the ac compressor unplugged
-lacking power

What I did:
-replaced air filter - no change but needed anyways
-checked and cleaned pcv valve and hoses - no change
-cleaned MAF - no change
-cleaned EGR - no change
-cleaned IAC - steadier idle, not so much surging but still a bit rough and larger amplitude surges when it did rev up
-Then i started feeling around the intake manifold blindly and found a the vacuum line running to the pcv valve on the left valve cover runs up into the bottom of the intake plenum just behind the throttle body. Theres a little rubber elbow that i had to get from the dealer for 30 bucks, ouch, that was completely caved in right at the 90. I took it off tonight and its not just caved in, its got a giant friggin hole in it. I can see right from the 90 to either exit of the elbow.

Part is in tomorrow, will install and let you guys know what the result is, but i'm betting its the culprit of most of my troubles if not all. Hope this helps others whom have looked for a vacuum leak but just can't find it.. it may be hiding from you under the intake plenum.

 

Replaced PCV Elbow (not PCV valve) - no surging of the idle, runs much better when driving. However, i still have a miss at idle which is especially bad when idling in drive. Will try some things and maybe compression check this weekend.

 

Replaced the plugs and wires today, and did a compression check while i was at it. Check turned out ok, #1 is down a little bit compared to others. Lite her back up and she idles like a new truck. I'm putting my money on the wires as the plugs i pulled out looked to be quite good/new condition. Next to do, find the broken power lock wire in the passenger door and replace the acc belt tensioner which is making a racket.